Automatic transmissions require an oil pump both for lubrication and for supply of control and actuation equipment, to supply the necessary quantity of oil and oil pressure in all operating states.
In older designs, the oil pump is generally directly or indirectly drive-connected to the drive motor so that the rotational speed, and consequently the pumping capacity of the oil pump, usually configured as a fixed displacement pump, are proportional to the rotational speed of the drive motor. In order to ensure the pumping capacity required to operate the transmission when a drive motor is idle, the oil pump is designed for the idling speed of the drive motor. This means that at higher rotational speeds of the drive motor, the oil pump will supply a multiple of the required quantity. Because of this, oil pumps of this type draw too much power, tend towards cavitation and produce relatively intense operating noise. Apart from that, the duct cross-sections have to be highly over-dimensioned. A further disadvantage is that no pumping capacity is available when the drive motor is halted so that the transmission is inactive and cannot be preconditioned for upcoming functions.
In order to meet steadily growing requirements on the transmission for spontaneity, comfort, and wear reduction, it has therefore already been recommended that in addition to the main oil pump driven by the drive motor, an electrical auxiliary oil pump is provided which supplies the necessary oil to the transmission even when the drive motor is stopped and supports the main oil pump when the drive motor is running at low rotational speeds. However, this represents a considerable design and construction effort as apart from the electric drive, a second pump, a connection of this pump to the existing hydraulic circuit, as well as a separate sensor system for switching the auxiliary oil pump is required. In addition, the accommodation of these additional components in the given installation spaces is often problematic.
From DE 197 50 675 C1, a drive device is well known in which the single oil pump is driven by an electric motor when the motor is idle or running at rotational speed below a given threshold, but above this threshold by the vehicle drive motor itself. In this known design, the electric motor and the oil pump are each arranged concentrically to one another and to the input shaft in a common housing, the oil pump is configured as a ring gear pump with a driven ring gear. The secure drive connection, between the electric motor rotor and the ring gear of the oil pump, is implemented such that the rotor and the ring gear are configured as an integrated component. Since the electric motor rotor is located inside the pressure compartment of the pump, tribologic problems arise at the external diameter of the internal ring gear, functioning as a slide bearing, due to the necessary enlargement of the ring gear, and variations in the seal gap occur during the temperature cycles owing to the pairing of different materials in the pump pressure compartment. In addition, when the materials of pump components are selected, their effect on the properties of the auxiliary electric drive has to be kept in mind, which restricts the choice of materials.
From DE 101 60 466 C1 a drive device is indeed known, but the stator and rotor of the electric motor are arranged outside of the pump housing. However, the drive connection, between the electric motor rotor and the driven internal gear of the internal gear pump, is not fixed, but is by way of a separate sheath-like connecting section that is rotatably mounted on the input shaft and is fixedly connected to the internal gear, and can be coupled to the input shaft and to the electric motor rotor, via a separate overrunning clutch. This arrangement does ensure the decoupling of both drive systems from one another in such a manner that the correspondingly inactive drive system need not be carried along by the active drive system, but it is relatively complicated in terms of construction and design owing to the additional overrunning clutch as well as to the connecting section joining the rotor located outside of the pump housing to the gear wheel inside the pump housing.
Against this background, the object of the present invention is to create a drive device, which is simple in design and construction and does not feature the disadvantages described above in connection with the electric motor disposed inside the pump housing.